This is supposedly derived from a report by The McKinsey group - but I looked around their website and couldn't find the report.

The claim is: that about a fifth of New York City residents are “early adopters” likely to purchase an electric car. Such vehicles could account for 16 percent of automobile sales by 2015, with 70,000 electric and plug-in hybrid electric cars on city streets.

But achieving a significant "carbon reduction" would require much more adoption than this.

“Large urban centers appear to be a natural environment for the emergence of ‘electromobility,’” stated the summary. “Currently, the world’s top 25 megacities (those with over 10 million people) contribute to about 10 percent of global CO2 emissions and, therefore, have a great need to reduce emissions.”